The most widely used type of pump in the oil and gas industry is known as the sucker rod pump. The pump is placed at the bottom of the downhole tubing near the reservoir. The pump is connected to an oil well pumping unit at the ground surface by a series of sucker rods with a polished rod and flexible cable above the ground. The oil well pumping unit moves the interconnected rods up and down, activating the pump and moving oil to the surface.
This oil well pumping unit includes a prime mover coupled via a gear reducer to a pair of crank arms that are rotated at one end about a fixed axis. A counterweight is mounted at the free end of each crank arm. A pitman rod is connected to each crank arm at one end and to an equalizer at the other end. The equalizer is connected to a walking beam that pivots up and down about a saddle pivot at the upper end of a sampson post. A horsehead is mounted on the front end of the walking beam allowing a flexible cable connection (bridle) to a polished rod which extends from the horsehead down into the well and is connected to the pump via the sucker rods.
In general, the conventional oil well pumping unit has used a random, inefficient linkage between the gear reducer and the polished rod where the primary consideration has been in meeting the stroke requirements for a given pumping unit. The conventional oil well pumping unit has the equalizer pivot directly above the crankshaft axis; it has no offset angle between the crank arm wrist pin line and the counterbalancing weight center of gravity line, and the ratio of saddle pivot-polished rod distance to saddle pivot-equalizer pivot is usually less than 1.4:1.
The disadvantages of the above described conventional pumping unit are an approximately 40% higher torque requirement than necessary, harmful gear reducer load reversals during portions of the crank arm cycle, a high upstroke rod velocity, rod stress, and rod fatigue failures. A further disadvantage in presently used pumping units is that they accommodate only one fixed size gear reducer, they have one specific structural capacity limitation, they have only two or three stroke length changes possible, and each length change is so far apart that there is virtually no fine tuning capability.
Conventional pumping unit design practices presently require seventeen API pumping unit sizes to accommodate a range of strokes between 88" and 168" and a range of gear reducer torques between 320,000 inch pounds and 912,000 inch pounds, and eighteen API pumping unit sizes to accommodate a range of strokes from 52" to 100" and a range of gear reducer torques between 114,000 inch pounds and 320,000 inch pounds.
In the prior art, McCray et al U.S. Pat. No. 3,371,554 discloses a connection between the pitman rod and the crank arm at only one of three discrete, spaced apart positions along an offset crank arm wrist pin angle line and also discloses a counterweight that adjusts to different positions along the crank arm. Miller et al U.S. Pat. No. 1,706,407 discloses a rack and gear arrangement to adjust the position of the counterweight along the crank arm.
Scherf et al U.S. Pat. No. 2,867,134 discloses an adjustable connection between the pitman rod and crank arm along the axis of the crank arm.